Light emitting semiconductor devices such as light emitting diodes may be fabricated from several materials systems, including III-phosphide, III-nitride, and II-VI materials. A “III-nitride” material system is any combination of group III and group V elements, with nitrogen being the primary group V element, to form semiconductors used in the fabrication of electronic or optoelectronic devices. This material system includes, but is not limited to, GaN, AlGaN, AlN, GaInN, AlGaInN, InN, GaInAsN, and GaInPN. The III-nitride material system is suitable for the fabrication of light-emitting devices (LEDs) that generate light with photon energies from the ultra-violet to the red spectral wavelength regimes. These LEDs include light-emitting diodes and laser diodes.
A III-nitride LED typically includes epitaxial layers deposited upon a suitable growth substrate to form a p-n junction via growth techniques, e.g. organometallic vapor-phase epitaxy. Though the examples described below have the n-type layers adjacent to the growth substrate and the p-type layers adjacent to the contacts, it is to be understood that the devices may have any arrangement of p-type and n-type layers. There are some unique challenges in the fabrication of III-nitride semiconductor devices. Because inexpensive III-nitride substrates are not commercially available, the epitaxial growth is forced to occur upon non-lattice-matched substrates, e.g. sapphire or SiC. The epitaxy-up orientation of the conventional LED die requires that light be extracted out the top surface, i.e. out through the contact layers. But, the high resistivity of p-type III-nitride contact layers, e.g. GaN, requires that metallization be deposited on the p-type material surface to provide sufficient current spreading. Because such metals absorb light, a very thin p-electrode metallization (e.g., Ni/Au) is typically used to allow light to escape through the top surface. However, even these thin semi-transparent layers absorb a significant amount of light. Assuming a typical thickness of 100 Å of Au and neglecting Ni (which may be oxidized to form transparent NiOx), the amount of light absorbed in this semi-transparent p-electrode is ˜25% per pass at λ=500 nm. At high current densities, the metallization thickness may need to be increased to maintain uniform current injection into the active region, and to avoid generating most of the light in the vicinity of the wirebond pad. Increasing the metal thickness increases light absorption and reduces the extraction efficiency of the device. Clearly, this tradeoff should be avoided in the design of III-nitride LEDs for operations at high current densities (>40 A/cm2, which is ˜50 mA into a ˜0.35×0.35 mm2 junction area).
In FIG. 1, Nakamura et al, in U.S. Pat. No. 5,563,422, disclosed a typical prior art III-nitride LED employing a sapphire substrate. Undoped and doped III-nitride layers surround an active region. A non-planar device geometry is necessary where contact to both p and n regions occur on the same side (top) of the LED since the substrate is electrically insulating. Also, two wirebond pads are required on the top of the device. The n-side wirebond pad is also an Ohmic electrode for making electrical connection to the III-nitride epi layers. The high resistivity of the p-type III-nitride layers requires current spreading to be provided by a thin semi-transparent (partially absorbing) NiAu Ohmic electrode that is electrically connected to the p-type III-nitride layers. Light extraction efficiency is limited by the amount of surface area covered by this Ohmic electrode and by the bonding pads. The optical losses associated with the Ohmic and bondpad metal layers are accentuated by the light-guiding nature of the III-nitride materials (n˜2.4) on the sapphire substrate (n˜1.8).
Inoue, et. al., in EP 0 921 577 A1, disclosed a prior art III-nitride LED having an epitaxy-side down or inverted structure where the light escapes predominantly upwards through a superstrate, i.e. the sapphire growth substrate. The device design conserves active junction area and provides for the smallest possible die size. The p electrode is made of Ni and Au, which are quite absorbing to visible light. Since this device lacks a highly reflective p-electrode metallization, it is limited in terms of light extraction efficiency and does not offer a significant improvement over the conventional (epitaxy-side up) device. Also, because the devices are small (<400×400 μm2) and use a small solder connection area to the package, they are limited in their light generating capability. Finally, this device suffers in efficiency from having guided light trapped within the III-nitride epi layers because of the low-refractive-index sapphire superstrate.
Kondoh et. al., in EP 0 926 744 A2, disclosed a prior art inverted III-nitride LED using a sapphire superstrate. The p-type electrode is silver, which is very reflective in visible light and results in a device with higher light extraction efficiency compared to the device disclosed by Inoue et. al. However, Ag adhesion to III-nitride material is poor. Upon annealing, Ag can conglomerate and destroy the integrity of the sheet Ohmic contact behavior and the reflectivity. Since the device is relatively small (<400×400 μm2) and uses a small solder connection area to the package, it is limited in its light generating capability. Finally, this device suffers in efficiency from having guided light trapped within the III-nitride epi layers because of the low-refractive-index sapphire superstrate.
Mensz et. al., in Electronics Letters 33 (24) pp.2066-2068, disclosed a prior art inverted III-nitride LED using a sapphire superstrate. This device employs bi-layer metal p-electrodes, Ni/Al and Ni/Ag, that offer improved reflectivity compared with Ni/Au. However, these devices exhibited high forward voltages of 4.9 to 5.1 V at 20 mA in 350×350 μm2 devices. This yields a series resistance of ˜100 Ω, which is more than three times higher than that of devices with good Ohmic electrodes. The high series resistance severely limits the power conversion efficiency. Since these devices are small (<400×400 μm2) and not mounted for low thermal resistance, they are limited in their light generating capability. Finally, these devices suffer in efficiency from having guided light trapped within the III-nitride epi layers because of the low-refractive-index sapphire superstrate.
Edmond et.al., in WIPO WO96/09653, disclosed a vertical injection III-nitride LED on a conducting SiC substrate, shown in FIG. 2. A conductive buffer layer is required for Ohmic conduction from the III-nitride layers to the SiC substrate. The growth conditions required for a conductive buffer layer limits the growth conditions available for subsequent layers and thus restricts the quality of the III-nitride active region layers. Also, the conductive buffer layer may introduce optical loss mechanisms that limit light extraction efficiency. Furthermore, the SiC substrate must be doped to provide high electrical conductivity (ρ<0.2 Ω-cm) for low series resistance. Optical absorption resulting from SiC substrate dopants limits the light extraction efficiency of the device. These conditions result in a trade-off between series resistance and light extraction efficiency and serve to limit the electrical-to-optical power conversion efficiency of the LED in FIG. 2.